The invention relates to metal/air fuel cells, and particularly to a hydrogen management system for closed system aluminum/air fuel cells.
Metal/air fuel cells or batteries produce electricity by the electro-chemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to air and to the aqueous electrolyte of the cell. During cell operation, oxygen is reduced within the cathode while metal of the anode is oxidized, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, zinc, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight and ability to function as anodes in metal/air fuel cells using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. A typical fuel cell unit or battery comprises a plurality of such cells.
Aqueous electrolytes for metal-air fuel cells consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
When a metal/air fuel cell is operated, quantities of hydrogen gas form as a parasitic by-product from the surface of the anode. As with other fuel cells or batteries this hydrogen can reach explosive concentrations.
Lapp U.S. Pat. No. 5,156,925 issued Oct. 20, 1992 describes a metal/air fuel cell in which hydrogen is collected and removed from a cell stack and discharged to the atmosphere. That system was intended for use in motorized vehicles and there was no problem in venting the small quantities of hydrogen to the atmosphere. It was only important to keep the hydrogen level in the fuel cell, vehicle and vehicle surroundings below the flammable limit of about 4% by volume.
However, when metal/air fuel cells must be used in a closed system where the hydrogen gas cannot easily be vented, it is necessary to convert excess hydrogen preferably to water. An example of a closed system is where the fuel cell is used as a power source in an unmanned underwater vehicle (UUV).
In a paper by Gibbons et al, "Closed Cycle Aluminum/Oxygen Fuel Cell With Increased Mission Duration" presented at the Power Sources Conference at in 1993, a system is described in which the hydrogen passing into a catalytic recombiner is maintained at a concentration of less than 3%. This is accomplished by employing oxygen as a carrier gas. However, there is no mention of what level of oxygen is used or how this level is maintained.
In George et al, U.S. Pat. No. 3,840,403 issued Oct. 8, 1974, a battery is described using a recombiner where the stoichiometric excess of one gas is stored for a period of time until a stoichiometric excess of the other gas is available for reacting with it to form water.
European Patent Publication 0 312 766 P1 published Sep. 2, 1992 describes a procedure in which the gas remaining after recombination is converted electrochemically at a gas consumable electrode.
It is an object of the present invention to provide a simplified chemical recombination process for combining oxygen gas with the excess hydrogen gas to form water.